1. Field of the Invention
The present invention relates to a photoelectric converter for use in an apparatus for optically inputting characters, symbols, picture images, etc., such as a facsimile, a digital copying machine or the like.
2. Description of the Prior Art
In a so-called line sensor in which a number of photosensor elements are arranged like an array, particularly in an elongated line sensor, it is important to securely separate the input signals from the respective photosensor elements while keeping a high S/N ratio and to transfer to the next signal processor, image producing apparatus and image transfer the signals apparatus.
However, it has been a difficult problem so far to satisfy the above-mentioned characteristic while realizing a low cost.
FIG. 1 is a circuit diagram of a conventional linear photoelectric converter and FIG. 2 is a set of timing charts for the circuit shown in FIG. 1.
In the diagrams a reference numeral 1 denotes a shift register for sequentially applying voltages to m common electrodes (B.sub.1, B.sub.2, . . . , B.sub.m); 2 indicates a photosensor array in which m.times.n photosensor elements are arranged in a line; 3 is a current amplifier for amplifying the photocurrents to be output from n independent electrodes (S.sub.1, S.sub.2, . . . , S.sub.n); 4 is a sample and hold circuit for memorizing and holding the output signals which were amplified by the current amplifier 3 and were converted into voltages; and 5 is a switching array for converting the output signals of the sample and hold circuit 4 into a serial signal.
The operation of the circuit shown in FIG. 1 will be described. When the i-th common electrode B.sub.i is selected by the shift register 1 and a voltage is applied to the common electrode B.sub.i, photocurrents flow through thc n photosensor elements (C.sub.i1, C.sub.i2, . . . , C.sub.in) in the i-th group connected to the common electrode B.sub.i. After the applied voltage response time of each photosensor element has passed, the signals amplified by the current amplifier 3 are memorized in the sample and hold circuit 4. Then, by sequentially driving the switching elements (SW.sub.1, SW.sub.2, . . . , SW.sub.n) of the switching array 5, the output signals of the sample and hold circuit 4 are converted into a serial signal and this serial signal is sequentially output.
On the other hand, a zero potential is applied to the common electrodes which were not selected.
In addition, the input potentials of the corresponding current amplifier elements of the current amplifier 3 are imaginary zero potentials and no voltage is applied to the photosensor elements connected to the non-selected common electrodes, so that no current flows. Namely, only the signals of the photosensor elements connected to the selected common electrodes are output to the corresponding independent electrodes.
In accordance with the operations as described above, the respective common electrodes B.sub.1, B.sub.2, . . . , B.sub.m are sequentially selected and the above operations are repeated, thereby enabling the photo information signals which were input to all photosensor elements to be output as serialized signals.
There are the following problems in the conventional photoelectric converter constituted by such a circuit.
(1) It is difficult to drive the non-selected common electrodes exactly at a zero potential, so that in the case of driving by an ordinary bipolar or CMOS IC, a potential of 10-50 mV is generated.
(2) An input offset voltage is generated in the current amplifier connected to the independent electrodes. This voltage is ordinarily about .+-.10 mV and is applied to the independent electrodes.
In the case of reading out the signals in such a situation, the potentials noted in the above items (1) and (2) are applied to the photosensor elements which are not inherently selected and to which the zero potential should be applied. Due to this, currents flow through these non-selected photosensor elements and these currents are added to the photocurrent signals of the photosensor elements which have been selected; therefore, a signal having crosstalk and a low S/N ratio is obtained.
In general, to solve this problem, a method has been used whereby blocking diodes for obstructing the crosstalk current are connected in series with the respective photosensor elements constituting the photosensor array. These blocking diodes are typically formed as Schottky diodes on the same substrate as the photosensor elements in order to reduce the number of steps for installation and to constitute a compact apparatus.
However, it is difficult according to the present technology to manufacture the diodes having uniform characteristics in the whole longitudinal direction but having relatively low reverse leak current with good yield. This difficulty causes the yield in the manufacture of elongated linear photoelectric converters to be remarkably reduced.